The electrophotographic developing method is a method including adhering toner in a developer on an electrostatic latent image formed on a photo conductor. As the electrophotographic developing method, a magnetic brush method using a magnet roll is widely employed in the present days. The developers for use in the method can be divided into two groups: a two-component developer composed of toner and carrier, and a one-component developer using toner only.
In a two-component developer, the carrier is mixed and stirred with the toner and has functions for triboelectrically charging and carrying the toner. In comparison with a one-component developer, a two-component developer has a better controllability in designing a developer. A two-component developer is therefore widely used in a full-color developing device that requires high image quality and a high-speed printer that requires reliability and durability in maintaining an image.
It is required for the two-component developer for such use to have image properties such as image density, fogging, white spots, tone reproduction and resolution at a predetermined level in the early stage. In addition, these properties are required to be stably maintained without fluctuation in an endurance printing period. It is therefore required for the developer to achieve high reliability as well as high definition and high definition image quality.
In recent years, from the viewpoints of high definition image quality and energy saving, toners such as polymerized toner and low-temperature fixing toner have been used. These newly developed toners have a problem of large environmental variation of electrical properties such as the resistivity and the charge amount when used in a developer. Accordingly, a developer having small environmental variation of electrical properties is required.
In Patent Literature 1 (Japanese Patent Laid-Open No. 08-22150), a ferrite carrier for an electrophotographic developer including a ferrite composed of MnO, MgO and Fe2O3, partly substituted with SrO, is described. It is said that according to the description in the Literature, by reducing deviation in magnetization among ferrite carrier particles, a ferrite carrier for electrophotographic developer that is excellent in image quality and durability and environmentally friendly, having a long life and high environmental stability, can be obtained.
In Patent Literature 2 (Japanese Patent Laid-Open No. 2013-50733), described is a ferrite carrier of electrophotographic developer having a core composition represented by a general formula MnxFe3-xO4+y (0<x≤1, 0<y) as main component, containing 0.1 wt % or more of Si and 0.03 wt % or more of Mg. It is said that from the ferrite described in the Literature, a carrier core material of electrophotographic developer which achieves high charging performance by itself and low environmental dependence can be obtained.
The inventions described in Patent Literatures 1 and 2, however, have difficulty to meet recent strict requirement for minimizing environmental variation of electrical properties such as resistivity and charge amount. Particularly in Patent Literature 2, although the improvement of the environment-induced fluctuation in resistivity is described in Examples, no specific suggestion is given on the improvement of the environment-induced fluctuation in charge amount.
In order to suppress environmental variation of the electrical properties, the surface state of a ferrite particle needs to be improved. Accordingly, in Patent Literature 3 (Japanese Patent Laid-Open No. 2012-25640), a ferrite particle represented by (MxFe3-x)O4 (M: at least one metal selected from the group consisting of Fe, Mg, Mn, Ca, Ti, Cu, Zn, Sr and Ni; 0≤x≤1) with the surface of ferrite particle body coated with alumina, and a carrier for electrophotographic developing including the ferrite particle having a surface coated with a resin are described.
According to the description in Patent Literature 3, it is said that a ferrite particle having a small apparent density and excellent fluidity can be obtained by coating the surface with alumina. In Patent Literature 3, however, no suggestion for improving environmental variation of electrical properties such as resistivity and charge amount is provided.
Also, in Patent Literature 4 (Japanese Patent Laid-Open No. 2014-137425), a ferrite particle including a complex oxide containing Fe and Mg in a solid solution state in the vicinity of particle surface, having different Fe and Mg contents between the internal part of a particle and the vicinity of particle surface, and a carrier for electrophotographic developing including the ferrite particle having a surface coated with a resin are described. Furthermore, in Patent Literature 5 (Japanese Patent Laid-Open No. 2014-182304), a ferrite carrier core material including practically Mg ferrite particle with a surface coated with a Ti compound, and a ferrite carrier including the ferrite carrier core material coated with a resin are described.
According to the inventions described in Patent Literatures 4 and 5, it is said that an Mg ferrite allows the resistivity and the magnetization to be controlled to any value without a surface oxidation treatment, from which a carrier excellent in charging properties can be obtained. Although these inventions achieve high charging capability, it does not mean that environmental variation of charge amount can be suppressed. Also, in Patent Literatures 4 and 5, no suggestion for improving environmental variation of resistivity is provided.
Furthermore, in Patent Literature 6 (Japanese Patent Laid-Open No. 2015-138052), a ferrite particle of practically Mg ferrite with a surface coated with a Ti compound, having an internal porous structure and an outer shell structure around the outer periphery thereof, and a carrier for electrophotographic developing including the ferrite particle impregnated or coated with a resin are described. It is said that the ferrite carrier according to the invention described in Patent Literature 6 has a low apparent density, which results in small stirring stress applied to toner and excellent charging stability in use of long duration. However, also in Patent Literature 6, no suggestion for improving environmental variation of electrical properties such as resistivity and charge amount is provided. In other words, the problem has not been solved by the invention.